INTRODUCTION

[5.01] This chapter considers illegality as a ground for judicial review. Illegality has been defined to mean �that the decision-maker must understand correctly the law that regulates his decision-making power and must give effect to it’.^[816] At its most obvious, this ground thus entails that a decision or other measure may be challenged where it has no legal basis (for example, where the decision-maker did not have the power that it purported to have^[817]); and it also means that a decision-maker’s failure to act may be challenged when the decision-maker is under a statutory duty to act in a particular way.^[818] Moreover, where a decision-maker has a statutory power and/or is entitled to make discretionary choices as to the performance of a statutory duty, it will act unlaw­fully if the power is exercised or the duty performed in a manner that is contrary to any one of a number of common law requirements.

[5.02] The chapter begins with a section that considers the importance of a number of �constitutional statutes’ that can found challenges not only to the decisions etc of public authorities, but also to Acts of the Westminster Parliament and of the Northern Ireland Assembly (viz, the European Communities Act 1972, the Human Rights Act 1998, and the Northern Ireland Act 1998). It next considers the core requirements of legality that apply to subordinate legislation; and it then outlines the range of related common law constraints that govern the exercise of discretion by statutory bodies, local authorities, government departments, and other �public’ authorities.^[819] The conclusion summarises the main points in each section.

[5.03] One further point by way of introduction is that the grounds for review—ille­gality, substantive review, and procedural impropriety—overlap with one another and should not be regarded as distinct.^[820] This therefore means that some of the developments discussed in this chapter are also of relevance to the other grounds for review and could alternatively be characterised as matters of substantive review or procedural impro­priety. The point is true not only in respect of the various common law requirements, but also of some of the requirements associated with the European Communities Act 1972 and the Human Rights Act 1998.